The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the use of multiple element surface coils and the reconstruction of images from MRI data acquired with such coils.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. An NMR signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
The NMR signal is typically received by a whole-body coil which is an integral part of the MRI system. However, in many applications the NMR signals are not strong and smaller coils which can be placed closer to the region of interest (xe2x80x9cROIxe2x80x9d) are used. Indeed, a multiple element (xe2x80x9cphase arrayxe2x80x9d) surface coil, or multicoil, is often used around a region of interest (e.g., thorax, head, abdomen, extremity) to increase the image signal-to-noise ratio (S/N). Reconstruction of the raw data from the xe2x80x9cNxe2x80x9d separate coil elements is typically achieved by a standard method in which a magnitude image is reconstructed from each separate coil element signal, and the resulting N images are then combined to form a composite image. This combined phased array magnitude reconstruction, or xe2x80x9csum of squaresxe2x80x9d technique, provides up to 90% of the maximum available S/N. However, this technique requires N separate two-dimensional Fourier transforms (2DFT) and magnitude calculation operations to produce the composite image.
Materials other than water, principally fat, are found in biological tissue and have different gyromagnetic ratios. The Larmor frequency of protons in fat is offset approximately 203 Hz. from that of protons in water in a 1.5 Tesla polarizing magnetic field B0. The difference between the Larmor frequencies of such different species of the same nucleus, viz., protons, is termed chemical shift, reflecting the differing chemical environments of the two species.
Often it is desired to xe2x80x9cdecomposexe2x80x9d the NMR image into its several chemical shift components. In the exemplary case of protons, which will be used hereafter for illustration, it may be desired to portray as separate images the water and fat components of the subject. One method of accomplishing this is to acquire two images S0 and Sxe2x88x921 with the fat and water components of the images in phase, and out of phase by xcfx80 radians, respectively (the xe2x80x9cDixonxe2x80x9d technique). Adding and subtracting these images provides separate fat and water images. The phase shift between the fat and water components of the images may be controlled by timing the RF pulses of the NMR sequence so that the signal from the fat image evolves in phase with respect to the water by the proper angle of exactly xcfx80, before the NMR signal is acquired.
In the ideal case above, the frequency of the RF transmitter is adjusted to match the Larmor frequency of the water. If the polarizing magnetic field B0 is uniform, this resonance condition is achieved through out the entire subject. Similarly, the out-of-phase condition (xcfx80 radians) for the fat component is achieved for all locations in the subject under homogeneous field conditions. In this case, the decomposition into the separate images is ideal in that fat is completely suppressed in the water image, and vice versa.
When the polarizing field is inhomogeneous, however, there are locations in the subject for which the water is not on resonance. In this case, the accuracy of the decomposition breaks down and the water and fat images contain admixtures of the two species. Field inhomogeneities may result from improper adjustment or shimming of the polarizing magnetic field B0, but are more typically the result of xe2x80x9cdemagnetizationxe2x80x9d effects caused by the variations in magnetic susceptibility of the imaged tissue, which locally distort the polarizing magnetic field B0. These demagnetization effects may be of short spatial extent but of conventional linear or higher order shimming techniques.
The influence of demagnetization may be accommodated, however, by a three-point Dixon imaging technique that uses three acquired images S0, S1 and S2, with the phase evolution times adjusted so that the fat and water components of the images are in phase, out of phase by xcfx80, and out of phase by xe2x88x92xcfx80 respectively. The complex pixels in each of the three images after conventional reconstruction may be processed as described, for example, in U.S. Pat. No. 5,144,235 to produce a separate water and a separate fat image.
In order to achieve robust and reliable water and fat separation, Dixon techniques employ extensive data processing for removing phase errors in the images. The robustness and the reliability of the Dixon method used can be compromised by the low SNR of the starting images. In addition, the processing time is significantly longer than conventional image reconstruction. These problems are exacerbated in imaging with phased array coils since the processing time is proportional to the number of coils N used, and the individual images acquired with phase array coils generally contain regions of very low SNR.
The present invention is a method for reconstructing an image from the NMR signals produced by a multicoil system using the xe2x80x9cDixonxe2x80x9d technique. More particularly, multiple Dixon image acquisitions are performed using a multiple element coil; a set of low resolution images and a set of high resolution images are reconstructed; the low resolution images are employed to calculate phase corrections that are made to the high resolution images; the corrected high resolution images are processed to form a set of water images; and the water images are combined into a single water image. The corrected high resolution images may also be processed to form a set of fat images which may be combined into a single fat image.